This invention relates to a four-stroke cycle engine unit to be incorporated in a portable working machine, and more particularly, to a valve drive mechanism of a four-stroke cycle engine unit for driving an exhaust and intake valves of the engine unit.
A portable working machine such as a lawn mower, a trimmer and a chain-saw is equipped with an internal combustion engine unit. In such a portable working machine, it is required for the engine unit to have a relatively compact structure and light weight because such a working machine is operated by hands of an operator and also required to be revolved with high rotation speed. It would also be better to be manufactured with a cheap cost. Such requirements may be satisfied by incorporating a two-stroke cycle engine in the machine.
In comparison of such a two-stroke cycle engine unit with a four-stroke cycle engine unit, the latter engine unit generates noise lower than that of the former engine unit and generates relatively clean exhaust gas, and the latter engine unit is operated with a reduced fuel consumption. In these points, the latter engine unit may be superior to the former engine unit, but the former engine unit can be constructed to be compact with light weight and driven with high speed. Because of such disadvantages, the four-stroke cycle engine unit is not suitable for the incorporation in the portable working machine such as a lawn mower. Namely, the four-stroke cycle engine unit has a size larger than that of the two-stroke cycle engine, which is a significant disadvantage for the portable working machine.